Active filters are normally realized with different techniques and topologies.
A known filter solution is, for instance, that of a RC active filter that uses a biquadratic cell with two operational amplifiers.
The characteristic parameters of a filter are the gain A0, the cutoff frequency ω0 and the quality factor Q.
Because of the technological variations of the components used (normally realized by integrated circuits) the above-mentioned parameters can vary and differ from their design nominal value. For this reason a control system to adapt their values is often necessary.
In the specific case of a RC active filter that uses a biquadratic cell with two operational amplifiers, and also in other cases, it is possible to modify the characteristic parameters of a filter by modifying three different components.
In other cases, for instance, for filters that use only one operational amplifier, the value of the three above mentioned parameters (A0, ω0, Q) depends on the same components, so that by modifying a parameter the value of the other two also changes.